"Noncontinuous" defined?

[dinos]

I understand "3 hours or more" for defining continuous loads, but is there a defined limit concerning "shorter duration" noncontinuous loads?

For example, I would expect that transformer inrush (lasting on the order of 0.1 seconds) might not be considered noncontinuous, but would motor inrush be considered noncontinuous?

 

[Dennis Alwon]

Motors have a 125% built in and in some case can have an overcurrent protective device that can be at 1100% in some cases depending on the type of motor etc.
So to answer your question, inrush has nothing to do with continuous load.

 

[infinity]

A continuous load operates at maximum current for 3 continuous hours or more. Anything else is non-continuous unless otherwise specified in the NEC as continuous such as hot water heater, 120 gals or less. {422.13}

 

[dinos]

What I am looking for is, is there a limitation on duration of the noncontinuous load?

If it is a very high value but only lasts 5 seconds for example, is it to be included in the load calculation?

 

[Carultch]

What I am looking for is, is there a limitation on duration of the noncontinuous load?

If it is a very high value but only lasts 5 seconds for example, is it to be included in the load calculation?

Unless there is a very high probability that all loads will spike at once, you will seldom include the spiking current in the load calculation. The time duration of what would count as a spike, and what would count as a load would likely be based on the time-response trip curves of all applicable overcurrent devices.

 

[winnie]

Educated guess rather than authoritative answer: I believe that if you are adding a 'non-continuous' load, for the purpose of sizing circuits and feeders, you have to count it as 100%, unless there is a specific allowance in the NEC that permits you to use a smaller power rating.

There are specific examples of this in the code, eg. circuit sizes for welders, supplies for X-ray equipment, circuits and feeders for electric ovens, etc. For these specific identified loads, the ampacity of the circuit/feeder and the VA used for calculations is lower than would be expected from the 'nameplate' wattage given on the device.

So your 100KW rodent vaporizer, which only operates for 5 seconds every time a rat gets caught would add 100KW to your load schedule, since the NEC doesn't know about rodent vaporizers. But a 25hp motor that puts 100KW of load on your system during starting gets counted as a 20KW load (round numbers), since the NEC knows about motors.

-Jon

 

[kwired]

What I am looking for is, is there a limitation on duration of the noncontinuous load?

If it is a very high value but only lasts 5 seconds for example, is it to be included in the load calculation?

Not trying to be too much of a smarty pants here but last I checked 5 seconds was much less then 3 hours

Motor starting current isn't included in branch circuit or feeder calculations, those are all based on full load current of the motor - and for most common general purpose motors this is taken from tables in art 430, if the motor doesn't fit a category in those tables then you use the nameplate full load current.

However one may take the surge of current at starting into consideration when it comes to what effects it may have on voltage drop to the rest of the system - but that is design issues not so much code issues and can be resolved with reduced voltage starting, soft starters, etc.

 

[dinos]

Not trying to be too much of a smarty pants here but last I checked 5 seconds was much less then 3 hours

Not trying to be too defensive but the question in my mind still is, can the non-continuous-ness be better defined?
We've got the upper end defined - why not define the lower end?

Something like, "loads lasting more than x seconds but less than 3 hours shall be considered noncontinuous".
In that way, whether or not the load is a lights, a toaster or a bug zapper or what have you, the time duration defines how it is applied.

I realize coming up with that lower end number may not be a simple affair, but I imagine there is logic/reasoning for the 3 hour upper end.

 

[RichB]

As I see it--the lower end is already defined--It is less than 3 hours--doesn't matter if it is .00001 sec or 2 hours 59 minutes and 59 secs--less than 3 hours is less than 3 hours

 

[dinos]

As I see it--the lower end is already defined--It is less than 3 hours--doesn't matter if it is .00001 sec or 2 hours 59 minutes and 59 secs--less than 3 hours is less than 3 hours

I admit to not having deep design experience, but I've seen typical 480/277V lighting panels with lets say a 200A main breaker, where they will include a branch circuit feeding a 30kVA transformer.

The primary FLA of a 30kVA is 36A.
Conservative transformer inrush is 12*36A=433A for 0.1 seconds.

Does that panel therefore violate the NEC requirements?

 

[GoldDigger]

I admit to not having deep design experience, but I've seen typical 480/277V lighting panels with lets say a 200A main breaker, where they will include a branch circuit feeding a 30kVA transformer.

The primary FLA of a 30kVA is 36A.
Conservative transformer inrush is 12*36A=433A for 0.1 seconds.

Does that panel therefore violate the NEC requirements?

As long as the wire size (which has a capacity for holding a short term overload without damage) is matched to the breaker size (which also has an ability to hold a short term overload on its thermal or electronic trip) the NEC is happy. Non-continuous current, for practical purposes, does NOT include short term overloads such as motor starting or transformer inrush.
As a practica matter, you do have to make sure that the time/current curve of the breaker is wide enough to allow things like motor starting under load, which can increase the length of the starting peak. For this reason the NEC specifically allows oversizing the breaker beyond the base 125% of FLA for some motor applications.

 

[kwired]

Not trying to be too defensive but the question in my mind still is, can the non-continuous-ness be better defined?
We've got the upper end defined - why not define the lower end?

Something like, "loads lasting more than x seconds but less than 3 hours shall be considered noncontinuous".
In that way, whether or not the load is a lights, a toaster or a bug zapper or what have you, the time duration defines how it is applied.

I realize coming up with that lower end number may not be a simple affair, but I imagine there is logic/reasoning for the 3 hour upper end.

I have to agree with others - lower end is anything less then 3 hours.

I imagine heat is the reason for the limit whatever it may be, when current flows in a conductor what resistance there is does cause some heating. Thermal elements of overcurrent protection devices also produce heat. (some 100% rated devices are solid state and have less internal heating others maybe just designed to manage heat differently - but those devices usually also require higher temp rated conductors because you still have some heat in the conductor) Cycling of a load does reduce the amount of heat produced where continuously running that load at a steady level does continue to add heat. Just a guess here - maybe they have determined that after about three hours that heat level doesn't increase to any significantly higher level in most typical applications.

 

[Electric-Light]

You're over thinking it.

Microwave ovens, toasters, hair dryers, etc = non-continuous. 99.9% of the time, these things don't see more than 20 minutes of continuous use. They don't factor in unreasonable use like trying to use a toaster as a space heater or using a microwave to boil bucket after bucket of water to fill a bath.

Computers, lights, TVs, air conditioners = continuous.

 

[infinity]

You're over thinking it.

Microwave ovens, toasters, hair dryers, etc = non-continuous. 99.9% of the time, these things don't see more than 20 minutes of continuous use. They don't factor in unreasonable use like trying to use a toaster as a space heater or using a microwave to boil bucket after bucket of water to fill a bath.

Computers, lights, TVs, air conditioners = continuous.

From your list, computers and TV's are non-continuous because they do not operate at their maximum load (part of the definition) for three straight hours. It could be anticipated that lights would be on for 3 hours or more. An AC could go either way, on one hand normally it would cycle on and off unless you put it in an extremely hot room where it had to run at it's maximum output for more than 3 hours.

 

[kwired]

You're over thinking it.

Microwave ovens, toasters, hair dryers, etc = non-continuous. 99.9% of the time, these things don't see more than 20 minutes of continuous use. They don't factor in unreasonable use like trying to use a toaster as a space heater or using a microwave to boil bucket after bucket of water to fill a bath.

Computers, lights, TVs, air conditioners = continuous.

Sort of right

your mentioned continuous loads could become non continuous if there were some control scheme that did limit their time or at least vary the peak current level more then 3 hours cycles anyway.

Most toasters probably will not run for 3 hours straight without a major failure, changing the bucket of water in the microwave "resets" the time and it is not a continous load, unless you need 3 hours + to heat a bucket of water.

 

[iwire]

Computers, lights, TVs, air conditioners = continuous.

Computers? Maybe but unlikely due to changing loads, hard drives and other hardware cycles.

Lights? Sometimes.

TVs? Sometimes

Air conditioners? Never

Take a look at the definition in article 100, notice the words maximum and expected.

 

[cowboyjwc]

Not trying to be too defensive but the question in my mind still is, can the non-continuous-ness be better defined?
We've got the upper end defined - why not define the lower end?

Something like, "loads lasting more than x seconds but less than 3 hours shall be considered noncontinuous".
In that way, whether or not the load is a lights, a toaster or a bug zapper or what have you, the time duration defines how it is applied.

I realize coming up with that lower end number may not be a simple affair, but I imagine there is logic/reasoning for the 3 hour upper end.

Well if continuous is 3 hours or more, then nocontinuous would be anything less. How much better do you need it?

 

[winnie]

I believe that the original poster's question could be rephrased:

"How does the NEC calculate short duration and intermittent loads? Is there an explicit definition for how to calculate loads of very short duration, such as startup transients or inrush currents?"

The 'continuous' versus 'non-continuous' question relates to the calculation of breaker size for a circuit ("100% of non-continuous loads + 125% of continuous loads").

As I mentioned in my previous responses, I don't believe that the NEC explicitly approaches this issue, but rather implies this with the sizing rules for different types of circuit. The rules for sizing the breakers for transformers deal with their inrush currents, similarly motor circuits are sized to accommodate starting currents.

However I don't think you would be permitted to serve any arbitrary intermittent load that is greater than the 'handle rating' of the breaker feeding the load. Say, for example, that you have a load that consumes 80A at 120V for at most 1 second. Unless specifically permitted elsewhere in the code, you couldn't place this load on a 20A breaker even though the breaker is specified to trip at 80A anywhere between 2 and 7.5 seconds.

-Jon

 

[kwired]

NEC does recognize "duty cycle" in some specific areas - some motor applications with a motor designed for a specific duty cycle otherwise all general purpose motors are considered to be continuous duty. There is also duty cycles for welding equipment.

 

[dinos]

Well if continuous is 3 hours or more, then nocontinuous would be anything less. How much better do you need it?

Consider a feeder feeding mixed loads...

Transformer inrush is often conservatively looked at as 12x the primary FLA for 0.1 seconds. I don't believe that current is used in calculation of 'noncontinuous load' in regard to feeder/protective device sizing.

An induction motor inrush might be 6 times its FLA and last for several seconds. I don't believe that current is used in calculation of 'noncontinuous load' in regard to feeder/protective device sizing.

If the above is true, then there is a lower limit to noncontinuous, which perhaps someday will be addressed in the NEC, and I think it would be better for it.